mplayer: disable for anything lower than ARMv5

Disable mplayer for anything lower than a v5 ARM core.
The support is broken and it's kind of pointless anyway. Fixes:
http://autobuild.buildroot.net/results/8f946e28b11b2ab1ba0d9688286c665488de0486/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>